The present invention relates to overclad fiber optic couplers that exhibit low polarization dependent insertion loss.
Overclad fiber optic couplers comprise an elongated matrix glass body through which optical waveguide fibers longitudinally extend. The diameter of the central region of the coupler is smaller than that at the ends of the coupler, whereby the fibers are more closely spaced and are of smaller diameter in the central region than they are at the ends of the unit.
Overclad couplers are usually formed by inserting into a glass tube at least a portion of each of a plurality of optical fibers so that the fiber portions occupy the midregion of the tube. The tube is evacuated, and its midregion is heated and collapsed onto fibers. The central portion of the midregion is thereafter drawn down to that diameter and coupling length which is necessary to obtain the desired coupling between the fibers. Couplers having various kinds of coupling characteristics, e.g. wavelength division multiplexer (WDM), achromatic, and the like, have been made by this process. See, for example, U.S. Pat. Nos. 4,931,076 and 5,011,251.
The term "overclad coupler" also includes couplers of the type made in accordance with the teachings of U.S. Pat. No. 4,799,949. A coupler preform is initially formed; it comprises a plurality of spaced glass optical waveguide preforms disposed in a body of matrix glass of refractive index n.sub.3. Each waveguide preform comprises a core and a cladding of refractive indices n.sub.1 and n.sub.2, respectively, where n.sub.1 &gt;n.sub.2 &gt;n.sub.3. The preform is stretched to form a uniform diameter glass rod in which optical waveguide "fibers" are embedded in the matrix throughout their lengths. The rod is severed into a plurality of units. The central region of each unit is heated and stretched to elongate it and taper it inwardly. Although completely embedded in matrix glass throughout their lengths, the waveguide paths are referred to as fibers since they possess optical fiber dimensions at the ends of the coupler.
Commercially available single-mode optical fibers usually have a value of n.sub.2 that is equal to or near that of silica. If silica is employed as the base glass for the tube, a dopant is added thereto for the purpose of decreasing n.sub.3 to a value lower than n.sub.2. The dopant B.sub.2 O.sub.3 (and optionally fluorine) has been employed to lower the refractive index of silica tubes. They also advantageously decrease the tube viscosity during the tube collapse step to a value lower than that of the coupler fibers, boron having the greater effect on viscosity. This enhances to a certain extent the collapsing of the tube onto the fibers. These dopants also affect the temperature coefficient of expansion (TCE) of the tube, boron increasing the TCE relative to silica, while fluorine reduces the TCE relative to silica when used in concentrations needed for couplers.
When the amount of B.sub.2 O.sub.3 (and optionally fluorine) in a silica tube is insufficient to soften the tube glass in 1.times.2 and 2.times.2 couplers, the tube glass excessively deforms the fibers during the tube collapse step, thus increasing the excess loss of the coupler. Silica coupler tubes have therefore contained a sufficient concentration of one or more of these dopants to provide an acceptably low value of excess loss. It was found, however, that overclad fiber optic couplers made from tubes containing such relatively high dopant concentrations exhibit an undesirably high polarization dependent insertion loss (PDL). In a 1.times.2 3 dB coupler made from a tube of SiO.sub.2 doped with 8 wt. % B.sub.2 O.sub.3, for example, (the dopant concentration being substantially uniform with respect to radius) excess loss is 0.28 dB and PDL is 0.39 dB. The term "substantially uniform" is used herein to mean that the radial concentration of a dopant does not vary from an average value by more than 0.5 wt. percent.
PDL is characterized as follows. Polarized light from a laser is introduced into the coupler input fiber. The insertion loss is measured at one or more of the output fibers as the input source is rotated with respect to the input fiber to cause the polarization of the input power to rotate through 180.degree.. The PDL is the difference between the maximum and the minimum measured insertion loss during this 180.degree. rotation of the input polarization. The PDL results from stress caused by a mismatch between the physical properties, i.e. softening point temperature and TCE, of the fibers and the tube. Cooling rate of a glass body also affects stress.
To more closely match the physical characteristics of the tube to those of the fiber cladding, and thus lower the PDL, the B.sub.2 O.sub.3 content throughout the tube can be decreased. In a 1.times.2 3 dB coupler made f rom a tube of SiO.sub.2 doped with about 3.5 wt. % B.sub.2 O.sub.3, for example, (the dopant concentration being substantially uniform with respect to radius), the PDL is decreased to 0.19 dB, but excess loss will have increased to 1.14 dB. As B.sub.2 O.sub.3 concentration in the tube is increased (while remaining substantially uniform with respect to radius), excess loss decreases, but PDL increases.